I build most sites using Joomla! and this post would have saved me months of not having to figure this stuff out on my own. However, I didn't even think about /images/ being in in the robots.txt. I use Sh404sef on every site. One thing that has really helped me improve my page speed score is a little plugin called jbetolo it has helped me improve my sites score by 20+ points by merging/compressing js and css as well as some other handy features. It does take some tweaking to make it work with other extensions/templates, but still well worth it in my opinion.